He's never been a college HC or coordinator. He's never even been the supervisor of a college FB position coach. In short, he has never done anything close to the job he would be asked to do as HC of a CFB program valued at $1,200,000,000.
If we were talking about USF, maybe you could make a case. Brian Hartline took that job. That program us valued at $103M, which is just 8.6% of the value of PSU. To put this another way, if you have a guy that is a novice pilot, does he first fly a cessna or does he fly Boeing 777? Would his first pilot job be flying small planes, or would United hire him to fly 400 people across an ocean? Why is that? The answer is experience and risk. That pilot has not demonstrated significant competence over time, so it is riskier. Terry has done an admirable job over the last 6 games, but he, like a new pilot, has not demonstrated significant competence over time, so he is a riskier choice than someone that has done the job, at any level, over time.
I'll give you another example. If you look through history, Governors have made better US Presidents than Congressman. Why? Because Governors are Executives, just like the President. Congressional representatives are NOT. They are 1 of 535. If Brian Harline is successful at USF (ie Governor of a State) he will get a shot at a P2 job in a few years.